All major industries use pipelines for transporting a wide range of liquids.
The use of devices that can be propelled by liquid flowing inside of a pipeline is well known, with such devices frequently described by the English term ‘pipeline pig’. The term ‘pipeline pig’, or simply ‘pig’, is commonly used by industry. Pigs are used when building a pipeline, as well as during the working life of such pipelines.
Pigs are typically comprised of a rigid metal body that serves as a support for at least two flexible scraping discs and/or cups, which function to propel the pigs, or which aid in scraping. Pigs may also be spherical.
Pigs may be used for filling or emptying pipelines, or to separate different products flowing in a single pipeline.
A frequent occurrence, depending on the flow inside the pipeline, that may complicate operating situations, or even lead to risks, is the formation of deposits on the inside wall of the pipeline. In some cases, these deposits may form very slowly, and may be soft and loosely attached. In such cases, they can be easily removed after they first occur. In other instances, incrustation may be more extensive, even eventually obstructing the flow of liquid completely. In both of the above cases, a pig is used to scrape off the material deposited inside the pipeline, cleaning it. Pigs are passed through the inside of the pipeline using a standard program that industry operators employ. This program varies in accordance with the severity of the deposit process.
The aforementioned pigs have been used for a long time, and are efficient when the inside diameter of the pipeline is constant.
However, a conventional pig may have certain drawbacks. One example is when the metal pig body inside the pipeline breaks. Pieces of the shattered body may become scattered inside the pipeline, or at pipe unions, or even in valves. Another possible example involves the ability of the pig to pass through very sharp bends in the pipe, which may cause a pig with a rigid body to snag.
Pigs made of non-rigid components are not subject to the above drawbacks. In the event of a structural failure, since the construction material is usually elastomeric, a second pig can always be passed to dislodge pieces of the elastomer that may be freely floating inside the pipeline and which the flow of liquid has not managed to dislodge.
Meanwhile, liquid transporting pipelines have recently gone into use with piping of variable nominal diameter. In these cases, the pigs so far used have proven to be inadequate, inasmuch as a pig may be built with a diameter sufficient for a specific inside diameter for one stretch of the pipeline, but which may lodge in another stretch with a smaller inside diameter, or the pig may lose its thrust if the inside diameter is larger, whereby the liquid flows through the annular space between the cup and the pipeline.
A typical example of a situation involving the above cases would be an elastomeric pig with cleaning cups of limited flexibility. Whenever the pipeline diameter reduces to a size consonant with the flexibility of the scraper cups, no problems will arise. However, if the diameter is considerably smaller, the flexibility of the cups will not allow them to change size sufficiently, and the pig will snag.
Thus the need has arisen to develop pigs capable of cleaning pipelines of varying diameters. Most innovations focus on cups adapted to the pig, in terms of the physical characteristics of the material they are made of, as well as the new configurations capable of responding to changes in diameter. Among specialists, these pigs are known as multisize pigs.
A very common and disadvantageous phenomenon involving pigs with so-called multisize sealers is that the sealers may become misaligned with regard to the pipeline axis (“nose down”), due to excessive flexibility of the sealers.
Another pig type within the current art is known as a “foam pig”, given this name because it is made of a polymer foam, for example polyurethane foam.
In comparison to the pig types as described above, a foam pig is characterized by its reasonable resistance and the ease of changing its shape.
Conventional “foam pigs” are shaped with a concave hollow in one of their ends, so as to act as a surface to concentrate the pressure caused by the propelling liquid, and with the other end having a more or less convex protrusion.
One characteristic of this type of pig is that they can change shape extensively. This change of shape makes them less efficient for removing hard deposits, and more readily scratchable due to the lower resistance of the material from which they are made, which may or may not cause them to get stuck inside the pipeline.
These drawbacks may be acceptable in the event of soft and easily removable deposits, and in view of the low cost of manufacturing these pigs.
The search for a more efficient cleaning of pipeline interiors using pigs made of this material has progressed. Proposals aimed at increasing the abrasiveness where the pig contacts the inside wall of the pipeline include modification of the surface texture of the pig body, creation of outer bristle inserts around the pig body, or a rough surface with various configurations, such as diamond shapes, e.g., in U.S. Pat. No. 3,602,934 (Acushnet Company) and U.S. Pat. No. 4,242,771 (Kenneth M. Knapp), or by overlaying different materials in other formats such as U.S. Pat. No. 5,895,619 (Knapp), or even by coating the entire pig with a layer of tiny bristles, metallic or otherwise, as shown in U.S. Pat. No. 4,016,620 (Pipeline Dehydrators, Inc.).
With regard to boosting cleaning efficiency, abrasiveness may pose risks to the inside wall of the pipeline, especially in the instance of flexible lines with a thin inner layer of stainless steel.
Proposals relating to ways to increase scraping capacity that have pointed toward bristles or changing the surface texture involve only certain thin strips, coiled around the length of the outside of the cylindrical pig body. As examples of this type of pig: U.S. Pat. No. 4,720,884 (T. D. Williamson, Inc.) U.S. Pat. No. 5,384,929 (TDW Delaware, Inc.) and U.S. Pat. No. 5,533,224 (Kenneth M. Knapp).
Thus the need arose for a pig that could move back and forth inside the pipeline, with scraper components able to produce higher contact tension with the inside wall of the pipeline, responding to changes in pig direction regardless of the reason, and without damaging the inside of the pipeline.